Method of producing solid composite board such as a door plate

ABSTRACT

A method of producing a solid board, such as a door plate, having a solid core made of porous, nonabsorbent particles compacted by adhesive means and having a first and a second core face, a frame surrounding said core adhesively connected thereto and having a first and second frame face flushing with said first and second core face respectively, a first and a second cover plate and reinforcement means being imbedded in said core, and having a uniform thickness substantially corresponding to the thickness of said core, THE STEPS COMPRISING: PROVIDING SAID FIRST COVER PLATE WITH A COATING OF AN AQUEOUS ADHESIVE SOLUTION ON ITS ONE FACE; PLACING SAID FRAME ON SAID COATED FACE AND ARRANGING SAID REINFORCEMENT MEANS ON SAID COATED FACE WITHIN SAID FRAME; FILLING THE SPACES OF SAID REINFORCEMENT MEANS WITH A NONCONGLOMERATED MIXTURE OF SAID POROUS, NONABSORBENT PARTICLES AND GRANULES OF ADHESIVE CAPABLE OF REACTING WITH WATER; PROVIDING SAID SECOND COVER PLATE WITH A COATING OF AN AQUEOUS ADHESIVE SOLUTION ON ITS ONE FACE; ARRANGING SAID SECOND COVER PLATE ON TOP OF SAID FRAME WITH ITS COATED FACE FACING SAID FRAME; AND PRESSING SAID TWO COVER PLATES TOWARDS EACH OTHER DURING HEATING, THEREBY PRESSING THEM AGAINST SAID FRAME AND SAID REINFORCEMENT MEANS AND ACTIVATING SAID GRANULES OF ADHESIVE BY STEAM EMINATING FROM SAID AQUEOUS ADHESIVE COATINGS.

United States Patent 1191 Snitker 1 1 METHOD OF PRODUCING SOLID COMPOSITE BOARD SUCH AS A DOOR PLATE [76] lnventor: Jens Christian Snitker, 4,

Related US. Application Data [62] Division of Scr, No 623,613, March 16, 1967, Pat.

[52] U.S. Cl. 156/284; 156/276; 161/162; 161/403 [51] Int. Cl .1 B321) 31/00 [58] Field of Search 156/284, 276; 161/44, 69, l61/68,403, 191,162

(56] References Cited UNITED STATES PATENTS 2,538,330 1/1951 Rosenhagen l. 161/68 2,745,779 5/1956 Ritter et a1 .1 161/403 X 3,328,218 6/1967 Noyes 156/276 X 3,364,097 H1968 Dunnington 161/403 X FOREIGN PATENTS OR APPLICATIONS 599,673 3/1948 United Kingdom u 161/69 Primary Examiner-William J. Van Balen Arrorney, Agent, or FirmErnest F. Marmorek 1 Aug. 5, 1975 157] ABSTRACT A method of producing a solid board, such as a door plate, having a solid core made of porous, nonabsorbent particles compacted by adhesive means and having a first and a second core face, a frame surrounding said core adhesively connected thereto and having a first and second frame face flushing with said first and second core face respectively, a first and a second cover plate and reinforcement means being imbedded in said core, and having a uniform thickness substan' tially corresponding to the thickness of said core,

the steps comprising:

providing said first cover plate with a coating of an aqueous adhesive solution on its one face;

placing said frame on said coated face and arranging said reinforcement means on said coated face within said frame;

filling the spaces of said reinforcement means with a non-conglomerated mixture of said porous, nonabsorbent particles and granules of adhesive capable of reacting with water;

providing said second cover plate with a coating of an aqueous adhesive solution on its one face;

arranging said second cover plate on top of said frame with its coated face facing said frame; and

pressing said two cover plates towards each other during heating, thereby pressing them against said frame and said reinforcement means and activating said granules of adhesive by steam eminating from said aqueous adhesive coatings.

2 Claims, 2 Drawing Figures PATENTED AUG 5 I975 l MiCiHOD OF PRODUCENG SOLID COMPOSITE BOARD SUCH AS A DOOR PLATE This is a division of my copending application Ser. No. 623,613, filed on Mar. 16, 1967 now US. Pat. No. 3,635,784.

This invention relates to a method of providing solid composite boards such as door plates.

While hollow doors may be used between rooms within residential flats or houses, other doors, such as external doors and doors leading from a flat to a staircase. are required to be solid.

Particularly for use onboard ships but also for use in residential houses and other buildings, doors and composite hoards, usually also approved as fire resistant, are known, which comprise a frame provided with surface sheets on both sides, the hollow space enclosed thereby being filled with a core consisting of particles, usually cork particles. united by means of a binding agent, which may be an adhesive, a ceramic binder, such as Sorel cement or the like, or, if the particles are partly expanded cork, resin originating from this cork. Such doors are relatively cheap, but owing to the flexibility of the core material, due to the use of the relatively soft particles, it is impossible to obtain complete planeness of the surface sheets, unless these sheets are relatively heavy, which will, however, increase the production costs.

The hollow doors most commonly used at the present time, comprise a frame provided with surface sheets on both sides, and a cellular reinforcement core made of fibrous material most frequently ofcardboard or paper. The cellular core may be of different designs, it may, for example consist of a large number of coil members arranged side by side without any interconnection, each member consisting of a strip, normally of paper, wound to form a spiral. Another commonly used cellular core consists of substantially zigzag-shaped cardboard strips, glued together in such a manner, that a large number of parallelogram shaped spaces result, and so that the core member can be of variable length, almost in the same manner as lazytongs, Irrespective of the design of such cellular cores, the strips of which they are built up are adhesively secured to the surface sheets along their edges, and thus the core serves to reinforce the surface sheets. The use of such cellular cores has made it possible, particularly in connection with mass production, to procure relatively cheap hollow doors having an adequate rigidity to prevent the surface sheets from giving way to pressure, even when relatively thin surface sheets are used.

The prime object of the present invention is to provide a method of producing a solid door plate or other solid composite board at low cost, consist mainly of inert materials, so that the board is certain to be nonwarping, even if it is exposed to humidity, and in which planencss of the surface sheets is secured even if relatively thin surface sheets are used.

According to the present invention such a solid composite board has been obtained substantially through a combination of the main features of the above mentioned known solid doors and of the hollow doors, likewise mentioncd above.

ln accordance herewith, in carrying out my present invention provide a solid composite board comprising a frame provided with a surface sheet on both sides, the space enclosed being filled with a core mass consisting of porous nonarbsorbent particles held together by adhcsive means, to which core mass the surface sheets are adhesively secured and in which core mass cellular reinforcement means known per se and made of strips of fibrous materials, such as cardboards or paper, are embedded.

This cellular reinforcement, the design of which is in reality of less importance, will secure a rigid interconnection of the surface sheets, so that, in spite of the flexibility of the core mass, fully plane surface sheets can be attained, even when these sheets are of very limited thickness. In addition, the core mass may be made from a relatively cheap material and, in the same way the known masses of this class, it is fully inert", so that the door will satisfy any requirement that may be made to a solid door, at the same time as it can be made at extremely low cost, also in comparison with the cheapest doors known up to now.

For production of the core mass a number of different porous, but non-absorbent, materials can be used, for example as previously known cork particles, but also granules of plastics materials or particles of expanded clay.

A particularly cheap solid door, or similar composite board, can be obtained, when the core mass is made from granules of bark containing resin or similar matter, preferably bark from coniferous trees, suitably bound together.

This means that the core material used in the door can be extremely cheap material, originating from waste materials is the disposal of which has caused trouble to the wood industries hitherto, the procedure being based on the experience that such granulated bark, when exposed to the combined effects of heat, compression, and moisture, is in a position to develop binding properties and unite into a solid block, provided the granulated bark is placed in an enclosed space of relatively limited dimensions. prior to the exposure to the said combined effects.

it is realized that also cork is a sort of bark, but a bark of quite special properties, deviating essentially from the bark obtained from trees other than the cork-oak. Hence, from the bark of the cork-oak granules of cork can be produced, consisting of particles of rather uniform size and having approximately equal dimensions in all directions, which properties cannot be obtained through granulation of other types of bark. These other barks, generally, will yield a mass consisting of relatively oblong particles of highly variable thickness, and whereas the bark of the cork-oak is stripped relatively easily, without any essential amount of sapwood adhering to the bark, granulated bark of any other origin, will usually contain a certain amount of sapwood. As a consequence, an attempt to make a board from granulated bark other than granulated cork without using so considerable amounts of binder that the bark granules amount to no more than a filling material in the binder, will result in a product that is highly fissured, possesses low strength, and has a surface of highly variable flexibility, that is a product without any applicability to practice.

If, on the other hand, granulated bark is used as a filling material for the cells or inter spaces ofa reinforcement. under the conditions described hereinbefore. it has shown up that the bark granules and the reinforcement will interbind into a continuous porous member which has no inclination to development of fissures, and which possesses a surprising uniformity in respect of resistance to influence from the outside. including resilience of the entire surface of the board produced. Possibly this is owing thereto that partly due to the interconnection between the reinforcement and the bark granules and due to the relatively narrow spacing of ad jacent strips of the reinforcement. compression of soft layers, if any, in the mass. is prevented, and partly to the reinforcement preventing the harder bark granules. when exposed to pressure. from pushing the softer granules together into more comprehensive layers or veins.

Another advantage of the invention is the possibility of producing low-cost solid doors or similar boards posessing also fire resisting properties. Thus, a door hav ing a core mass based on granulated cork or granulated bark will possess fire resisting properties, to such an extent that a door of a thickness of approximately 35 mm will satisfy the normal requirements to a so-called 30 minutes fire resistant door.

A solid door. possessing particularly good fire resisting properties, and being of extremely low cost at the same time, is obtainable, however, provided that the core mass consists of granules of vermiculite or a like mineral matter expanded through heating. held together through a binder.

Hereby the core mass itself will be non-imflammable and, since it possesses also prominent heat-insulating properties, such a board will be able to resist the effects of direct fire for a considerably longer time than corresponding boards having a core mass based on cork or other materials of organic origin which. when exposed to the effects of fire, will be slowly carbonized and burn away. A condition for such expanded mineral matter being applicable to a core mass for solid door or similar board is, however, that the reinforcement described hereinbefore is embedded in the mass. thus involving that the mass consists of a large number of prismatic components, shape-stable in themselves, held together solely by the reinforcement member; this shapestability would not be obtainable if no reinforcement is used. The binding of the granules through such binder involves not only that the granules, also in the case of the surface sheets of the board burning away. remain intact combined into a solid mass but also that flames and hot gases cannot penetrate through the material and thereby indirectly spread the fire to the side of the board opposite to the side exposed to direct fires.

Whereas the reinforcement has proved to be necessary with a view to achieving a satisfactory board. this reinforcement constitutes nevertheless a weak point seen from a fire-fighting point of view. since the reinforcement or some of the strips thereof will be carbonized when exposed to strong heat. and although the carbonized fibers are partly held in place as a consequence of the adherence to the granules, there is the possibility nevertheless that. after expore to fire for some time throughgoing passages may develop in the board. through which passages fire may penetrate to the side of the board opposite to the side exposed to direct fire.

This development of passages can be essentially delayed through application of a coating of a fire resistant impregnating compound. such as water glass or ceramic glue. on the reinforcement member. Such impregnation is without any effect on the yieldability of the strips of the enforcement in their transversal direction. which is a condition for the adaptation to the mass consisting of granulated material. but it entails an cssential delay of the burning through of the reinforcement strips.

Such impregnation is not strictly necessary. however. when the materials constituting the core mass. such as is the case when. for example. the mass is based on cork or bark, expands at temperatures above 300 C. Although. also in this case the reinforcement strips. made of fibrous materials. will soon be carbonized. so that there might be a risk that passages going through the mass would result. which passages might lead the fire from one side of the door to the other. Experience has shown. however. that the particular fire resistant property of an expandable mass also serves to prevent development of such passages. as the layer remaining after the carbonization of a fibrous strip having proved to be so soft and weak that the expansion resulting from the heating is sufficient to involve that the granules on the two sides ofa strip are pressed through the carbon ized strip and against one another. thereby closing the passages effectively again. so that the board possesses approximately the same fire resisting properties as a board without any reinforcement.

if, on the other hand, the core mass is based on ver miculite or other material not expanding by heating. a similar effect can be obtained. however. through addition to the mass of a powdery or granulated expansive material expanding by heating to temperatures above C. Such material. of which. for example. a proportion of 2 70. by volume, of the quantity of vermiculite or corresponding expanded mineral material. may be added. causes the core mass to expand by heating. whereby. in turn. the granules of two prismatic members of the mass adjacent to a wall constituting part of the reinforcement. will penetrate. when the wall is carbonized. through the coke developed. so that the two prismatic members of the mass come into contact and prevent development of through passages owing to burning of the reinforcement strips.

This expansive material may, for example, be a mixture of ammoniumdiphosphate and dextrin. for example equal parts of these two compounds. The expansive material may also be granulated vermiculite raw material. which will expand at a rate increasing with the temperature. and which will only reach full expansion at about 900 C. whereby it will be of a volume of fifteen times the initial volume. Since during the manufacture of a door or a similar board of the type in question the board is normally heated to a temperature slightly above 100 C to promote the binding process. the expansive additive must, however. be of such a nature that no essential expansion will result until the temperature has reached above l00-l20 C.

It may be advantageous. that at least one of the surface sheets is made of a material which is in itself highly fire resistant. for example asbestolux. or plywood which has received special treatment. A surface sheet of this type. provided it is placed on the side of the board exposed to the fire. will in itself essentially increase the time passing before the other side of the board reaches a temperature involving any risk of spreading of the fire. and in case it is placed on the other side of the board. even if the surface sheet on the side exposed to the fire has been burnt off, or burst off. it will serve to keep the core mass together. also in case of the reinforcement strips being more or less carbonized. and even in the case of through passages having been developed in the mass. because the risk of spreading of the fire involved by such through passages is relatively low, as long as these passages are not open on the side of the board not exposed to direct fire.

Just the circumstance that only the use of one single surface sheet of strongly fire resistant material involves a considerable increase of the fire resisting capacity of the board, involves, moreover, the possibility, of using different materials with different resonant frequency for the two surface sheets, which to facilitate the manufacture should preferably be of equal thickness, whereby is obtained that the sound insulating capacity of the door is considerably higher than would be the case if the two sheets were of the same material, and had the same resonant frequency.

A special problem is connected to the embodiment of the reinforcement member in the mass, in such a manner that the strips forming the reinforcement, in any case over the greater part of their length, retain their width which determines the spacing of the surface sheets.

The material previously used for the production of hitherto known core masses has consisted of a mixture of the granulated material and an aquous adhesive solution, or a slurry of a ceramic binder, and if such material is poured over the reinforcement and then brought down into the cavities thereof, the moisture from the mixture will to some extent involve soaking of the reinforcement strips, so that the strips may be deformed when the mass is pressed into place, and, after the filling in of the mass, their edges will no longer be in the same plane.

This disadvantage may in some extent be remedied by treatment of the reinforcement strips with plastic, for example. The reinforcement thus treated will, how ever, be considerably more expensive than a nontreated reinforcement and, moreover, being less flexible than a non-treated member, whereby the work involved in fitting and adaptation of the reinforcement is increased. Said disadvantage may also be remedied through the use ofa reinforcement made of strips hving a width somewhat in excess of the intended spacing of the surface sheets, the mass with embedded reinforcement thus resulting being subsequently ground down to plane. The use of such treated reinforcement strips of grinding, will involve increased production costs, however.

Therefore the present invention also relates to a method of production of a solid door of the type in question, according to which for the fixing of the surface sheets, is effected, in a manner known per se, an aquous adhesive solution is used. which is applied to the inside of the surface sheets, and the frame and the reinforcement are placed on the side of the one surface sheet, thus coated with adhesive, whereafter the hollow spaces of the member are filled with granules mixed with a binding material reacting with water and having a water contents so low that the binding material is still a powder, whereupon the other surface sheet is placed on the upwards facing side of the reinforcement, and the board as a whole is subjected to compression and heating.

The term binding material reac ing with water" shall here include such binding materials, like adhesives or, for example, ceramic binders. as require addition of water in order to develop binding or adhesive properties, the water being. for example for the adhesives, a

condition for the adhesives being in a position to adhere to the surface of the granulated material.

The said method is based on the experience that the aquous adhesive solutions generally used in wood industries, the active compound thereof usually being based on plastics, contain so much water that owing to the heating the amounts of adhesive applied to the surface sheets, are able to give off the amount of water required for the binding of the granules of the core mass and for bonding of the granulated mass and the reinforcement together, the water being given off primarily as vapour that easily penetrates through the mass not yet hardened, but, due to the delay in the heating of the mass, the vapour condenses into water which is absorbed by the binding material and initiates the effect thereof.

Thus, the above said method involves the possibility of filling the hollow inter spaces of the reinforcement placed on one of the surface sheets, with a material that is essentially dry, so that no soaking and softening of the reinforcement strips results and filling of teh cells or inter spaces is obtained with certainty, since the hardened core mass is not brought about until the application of pressure and heat.

The granules mixed with binder constituting the core mass, can be placed in a layer, somewhat thicker than the width of the strips of fibrous material constituting the reinforcement, so that a compression of the mass results during the fixing of the surface sheets, owing to the pressure used for this operation. Since the strips constituting the reinforcement are thin and rigid, the granules to be found over the edges of the strips at the time in question will move down alongside the strips on both sides thereof, without the edges being damaged. The compression involves at the same time, however, that non-uniform filling of neighbouring inter spaces in the reinforcement is adjusted through deformation of the relevant walls of the reinforcement, which may be broken, if necessary. It appears, however, that the de formation develops in the longitudinal direction of the strips, however, and there is no or insignificant deformation transversally thereof, so that, also after the compression, the strips have the same width throughout, and thus secure a constant spacing of the two surface sheets.

When the mass is made from bark, it is not strictly necessary, that a special binding material is added. Thus, a board based on bark, can be produced by drying the raw bark until it contains about 20 of water; granulating the bark and filling the material thus granulated into the inter spaces of the reinforcement arranged in the frame, placed on the one surface sheet coated with an aquous adhesive solution, placing. thereafter, the other surface sheet on the upper surfaces of the frame and the reinforcement; and pressing said components together, the compression being combined with heating.

Experience has shown that drying of the bark prior to the granulation. to the contents of moisture required, highly facilitates the process of granulation, at the same time as a relatively uniform moisture throughout the granulated mass is attained, whereas this uni formity of moisture contents has appeared to be difficult to attain if the adaptation of the moisture is undertaken subsequent to the granulation, whether this be undertaken on bark of higher moisture contents or on bark dried to essentially lower moisture contents.

It should be noted that, also in connection with this last procedure, it may in certain cases be convenient to add more or less powdered binding material. It is essential, however, that the bark-material. with or without addition of powdered binding material. at the time of being filled into the reinforcement, is dry to such an extent that there is no inclination to development of lumps, due to adhesion between the particles resulting from the presence of moisture.

Other aspects, objects and the several advantages of this invention will be apparent from a study of this dis closure. the accompanying drawing and the appended claims.

In the drawing .FIG. 1 is a fragmentary sectional front view of a door embodying the invention not yet finished and showing a frame placed on a surface sheet and an enforcement placed within said frame, prior to being filled. and

FIG. 2 is a fragmentary sectional front view of the finished door.

FIG. 1 shows a part of a frame, consisting of a longitudinal frame member 10 and a transversal frame member 12, the frame members being placed on a lower cover plate or surface sheet 14. Inside the frame 10,12 is on the upwards facing side of the surface sheet 14 placed a cellular reinforcement 16 made of strips of fibrous material. usually cardboard. and which, in the embodiment shown, is of the type composed by a num ber ofthin cardboard strips 18, having a width equal to the thickness of the frame members 10 and 12, and so interconnected that parallelogram-shaped inter spaces or cells 20 are formed between them.

Prior to the placing of the frame 10,12 and the reinforcement 16 on the lower surface sheet 14 the entire upwards facing side thereof has been coated with an aquous adhesive solution, preferably an aqueous solution of a plastic adhesive, such as ureaformaldehyde glue.

The components referred to are thereupon placed under and moved relative to a mixing and feeding device working approximately on the principles of. for example, a broadcast sower, from which a steady stream of thorougly mixed granulated material, preferably granulated cork. granulated bark. or granulated vermiculite. and a binding material, for example powdered ureaformaldehyde, having such moisture contents that it remains as powder and thus is conveniently mixed with the granulated material, is discharged over the frame 10,12 and the cellular reinforcement 16 arranged therein. The volume of the amount of binding material will, normally, be between 5 and 15 70 of the volume of the amount of granulated material. The rate of advance of the components is so adapted that the mix of granulated material is deposited in a layer that is somewhat thicker than the width of the strips 18 which are all of the same width. Thereafter is removed from the top of the frame 10,12 any deposit of granulated material. for example through manual sweeping. whereupon the other cover plate or surface sheet 22, see FIG. 2, which likewise is coated, on the underside. with an aquous adhesive solution. is placed on the upper surface frame 10,12 with contents.

Thereupon. the frame 10,12 with the two surface sheets 14 and 22 is placed in a heated press. which may be either a plane press. covering the entire area of the surface sheets 14 and 22 or a roller press. so that the surface sheets 14 and 22 are brought into contact with the frame 10,12, and the core mass contained therein compressing the granulated materials of the same time.

Due to the heat. the moisture from the adhesive coatings applied to the sheets 14 and 22 will penetrate through the entire mass between same. whereby the binding material contained in the mass will be activated and binding of the granules into a solid mass. in which the reinforcement 16 is embedded, results.

As is indicated, on an exaggerated scale. in HQ. 2. this involves to some extent a deformation of the strips 18 of the reinforcement, and fracture thereof may ap pear in some cases. as indicated by 24, without any change of the width of the strips 18, however.

On completion of this compression operation. a completed door plate has been produced. the surface sheets 14 and 22 thereof being held at a constant mutual spac ing by means of the reinforcement 16, this spacing being equal to the thickness of the frame members 10 and 12, so that the two surface sheets will both have plane outside surfaces.

The sheets 14 and 22 may each be of any desired material, such as plywood. wood fibre board. asbestolux. or similar kinds of board. which can be relatively thin. the thickness being for example 2 3 mm.

After the binding and gluing process, the sheets 14 and 22 and the edges of the frame 10.12 are cut to size. and a thin decorative veneer of high quality wood. metal. or other desired materials, may be applied on each surface sheet. Such decorative veneer may. of course. be omitted in cases where the door is intended to be painted.

1n the embodiment shown the frame members 10 and 12 are relatively wide. These frame members may also be more narrow. In such cases it will be appropriate in a manner known per se to provide strengthening members, where the hinges are to be fixed, for which mem bers corresponding cut outs must be made in the reinforcement. Apart herefrom such strengthening members involve no changes to the procedure described. Corresponding strengthening members may be arranged. where the door-lock is to be placed. although this should not normally be necessary. since the recesses required for insertion of the locking device are easily shaped in the core mass.

In the embodiment shown. the reinforcement is con stituted by a continuous system with parallelogramphaped spaces between the strips of cardboard or other fibrous material, but any other shape of such reinforcement may be embedded in the core mass. for example the so-called paper-spirals. The reinforcement shown involves the advantage. however. that. it is quickly installable, at the same time as it can be expanded to a higher extent than is normally the case in connection with manufacture of hollow doors. because. in a solid door the reinforcement serves only to secure exact spacing of the two sheeting boards. whereas the core mass itself contributes essentially to the establishment of the rigidity required. The reinforcement can thus have relatively large inter spaces. so that it constitutes a relatively inconsiderable part of the total filling between the surface sheets.

1 claim:

1. A method of producing a solid board. such as a door plate. having a solid core made of porous. nonabsorbent particles compacted by adhesive means and having a first and a second core face. a frame surrounding said core adhesively connected thereto and having a first and second frame face flush with said first and second core face respectively. a first and a second cover plate and reinforcement means being imbedded in said core and having a uniform thickness substantially corresponding to the thickness of said core.

providing said second cover plate with a coating of an aqueous adhesive solution on its one face; arranging said second cover plate on top of said frame with its coated face facing said frame; and pressing said two cover plates towards each other during heating, thereby pressing them against said frame and said reinforcement means and activating said granules of adhesive by steam ema nating from said aqueous adhesive coatings. 2. A method as defined in claim supplying said mixture in a layer slightly thicker than the said frame thickness and removing granules thereby deposited on the upwards facing of said two frame faces before said arranging of said second core face. 

1. A METHOD OF PRODUCING A SOLID BOARD SUCH AS A DOOR PLATE HAVING A SOLID CORE MADE OF POROUS NON-ABSORBENT PARTICLES COMPACTED BY ADHESIVE MEANS AND HAVING A FIRST AND A SECOND CORE FACE A FRAME SURROUNDING SAID CORE ADHESIVELY CONNECTED THERETO AND HAVING A FIRST AND SECOND FRAME FACE FLUSH WITH SAID FIRST AND SECOND CORE FACE RESPECTIVELY A FIRST AND A SECOND COVER PLATE AND REINFORCEMENT MEANS BEING IMBEDDED IN SAID CORE AND HAVING A UNIFORM THICKNESS SUBSTANTIALLY CORRESPONDING TO THE THICKNESS OF SAID CORE, THE STEPS COMPRISING: MIXING SAID NON-ABSORBENT PADRTICLES WITH GRANULES OF AN ADHESIVE CAPABLE OF REACTING WITH WATER TO PROVIDE A NON-CONGLOMERATED MIXTURE, PROVIDING SAID FIRST COVER PLATE WITH A COATING OF AN AQUEOUS ADHESIVE SOLUTION ON ITS ONE FACE, PLACING SAID FRAME ON SAID COATED FACE AND ARRANGING SAID REINFORCEMENT MEANS ON SAID COATED FACE WITHIN SAID FRAME, FILLING THE SPACES OF SAID REINFORCEMENT MEANS WITH SAID NON-CONGGLOMERATED MIXTURE, PROVIDING SAID SECOND COVER PLATE WITH A COATING OF AN AQUEOUS ADHESIVE SOLUTION ON ITS ONE FACE ARRANGING SAID SECOND COVER PLATE ON TOP OF SAID FRAME WITH ITS COATED FACE FACING SAID FRAME AND PRESSING SAID TWO COVER PLATES TOWARDS EACH OTHER DURING HEATING THEREBY PRESSING THEM AGAINST SAID FRAME AND SAID REINFORCEMENT MEANS AND AVITVATING SAID GRANULES OF ADHESIVE BY STEAM EMANATING FROM SAID AQUEOUS ADHESIVE COATINGS.
 2. A method as defined in claim 1, supplying said mixture in a layer slightly thicker than the said frame thickness and removing granules thereby deposited on the upwards facing of said two frame faces before said arranging of said second core face. 